Art of cooling and moderating neutronic reactors



Sept. l, 1959 M. KosMlN EI'AL 2,902,425

ART 0F COOLING AND MODERATING NEUTRONIC REAcToRs Filed June 7, 1956 um.um,

IN VEN TORS.

/17/4 roA/ KosM//v BY /W/ucoav /VfvE/v United States Patent- ART OFCOOLING AND MODERATING NEUTRONIC REACTORS Milton Kosmin, Dayton, andMalcolm McEwen, Mialnishurg, Ohio, assignors, by mesne assignments, tothe United States of Americalas representedbythe United States AtomicEnergy,` Commission Application June 7, 1956, Serial No. 590,002

' `6 Claims. (ci. 2in-193.2)

U-235, occurring in natural uranium to the extentof one part in 139parts of natural uranium could be tissioned'by bombardment with thermalneutrons, resulting in theproduction of two lighter elements havinggreat kinetic energy, together with approximately two fast neutrons onthe average together with beta and gamma radiation.

Vast amounts of heat energy are liberated in this-reaction and therecovery and use of such heat has presented attractive possibilities asa source of nuclear power.

The practical generation and recovery of the nuclear or atomic heat soliberated was, of course, dependentV upon the successful solution of theproblem `of safely inducing and controlling a self-sustainingl chainreaction.

As is Well known to those skilled in this-art, this Vproblem was solvedby arranging bodies of the. iissionable material,l

usually uranium or enriched uranium, in a geometric pattern within amass of moderator in such fashion-that a self-sustaining controllablechain reaction was obtained. The considerable amounts of heat generatedinthe'bodies of ssionable material were removed either by cooling thesebodies with a gas or with aWsuitable-liquid;` Asaresult there wasdeveloped two general types of neutronic reactors which came to berefererd to as gas-cooled and liquid-cooled` reactors.

For the purpose of# recovering the yheat liberated by the fissionedchainreaction and utilizing such heat in a heat engine of conventionaltype, the liquid-cooled reactor has receivedthe greatest attention andit is with this type of reactor thatthis' invention is concerned..

Methods for constructing and operating neutronic reactors for carryingout`the chain reaction are Well known in this art and are described, forexample, by Fermi and Szilard in U.S. PatentV No. 2,708,656, issued May17, 1955. This patent' is by reference incorporated herein and made apart of this disclosure. According` to the disclosure of this patent,either light water, H2O, heavy water, D20, or diphenyl (biphenyl) may beused as a moderator andcoolant in the liquid-cooled reactor.

Attractive possibilities are presented by the use of biphenyl as areactorV coolant. The properties of this material, i.e., itsrelativelyhigh boiling point at atmospheric pressure (255 C.), itschemical composition consisting only of carbon and hydrogen, and itsthermal stability makepossible the` operation of reactors cooled withthis material atltemperatures as highas 425 C., or higher, forextendedperiods of time. ,Amrajor drawback encountered in the use ofthismaterial lies in its relatively high freezing point (70 C.) and the factthatsome polymerization takes place in the biphenyl` asa result ofradiolytic damage.

Pice

We have now foundthat monoisopropylbiphenyl can be used as a reactormoderator and coolant in place of, or in admixture with, biphenylr(employing in s uch` mixture, however, not in` excess of 20% by weightof biphenyl) and all of the advantages hitherto obtained-by the use ofbiphenyl 'are realizedbut in greater degree and and without encounteringthe'ditliculties Vattendant upon the use of a material of high freezingpoint.`

Monoisopropylbiphenyl may be .readily prepared by alkylating biphenylwith propylene using A1Cl3 as a catalyst. When so preparedthe productusually consists-of a mixture of the meta and para isomers. The ratio ofmeta to para isomer in the isopropylbiphenyl may be varied somewhatby"changing the' amount ofA'lClg c'at'- alyst employed during thealkylati'on. In general, however, the successful use of this materialdoes not depend upon any particular ratio of isomers since both the metaand para isomers are equally resistant to radiol'ytic damage and areequally effective neutron Amoderators. However, in order to obtain a lowmelting liquid coolant it is desirable to provide a mixture of themetaand para-isopropylbiphenyl in which the contentI of either isomerislnot lower than 10% by weight of the mixture.

The following example illustrates the. preparationioff a mixture ofmetaand para-isopropylbiphenyl suitable for use as a coolant in aneutronic reactor.l

Eighty pounds of biphenyl crystals were charged to an acid-resistantalkylation kettle equipped with an agitator, inlet and outlet means, athermometer and-a heating and coolant jacket surrounding the exterior ofthe.

kettle. The biphenyl charged was meltedin the kettle and 0.70 pound ofanhydrous A'lCl'a added, the reactor charge heated to -80" C. and thensaturated with HCl by the addition of approximately 0.4 pound:r oft an"-hydrous HC1. The propylene gas feed was then started and maintained atarate suiiicienttofkeepfa small' outl ilow of gas from the outlet of thealk'ylation: kettle. During this time cold Water was passedinto'fthe'jacket surrounding the body of ther` alkylatoriata'ratesoaslto maintain the reacting mass at a temperature of 75-80" C.Afterl^3.25 poundsm of propylene" had-been taken* up in the reaction,the supply of this gas was discontinued. The reaction-mass' was cooledto50C.

Five gallons of distilled water was added slowlyf at rst, the mixturestirredforlO minutes, heated to 20 C. and then transferred. to-aseparating-tank. 'Ilheaqueous phase was drawn oi` and discarded. Theorganic layer was then successively washed first with 5 gallons of watercontaining 5% NaCl plus 1 liter of concentrated HCl, then with 5%aqueous NaCl solution, then 55% aqueous NaCl solution containing -5f%fof' NaHCOg and finally with 5% aqueous NaCl solution.

The washed organic liquid resulting from the above washing step was thentransferred to a fractionating column and fractionated first to removethe unreacte'cl` biphenyl which was done-at a column head temperature of13T-138 C. at 24 m'm., then an intermediate fraction at a headtemperature of 13C-160 C. at 17 mm.s, and finally monoisopropylbiphenylwas recovered at a head temperature of 1GO-169 C. at 17 mm.s. Theproduct amounted to` 32 pounds. The melting point of the product was.-47 C. The product contained 38% byy weight of para isomer and 62% byVweight of meta isomer. The boiling point of the mixture at 7'60 mm.swas 290 C.

A representative sample'of the product prepared as described abovewas-exposed at a temperature up to 585 F. to intense neutron bombardmentin ,a neutronic reactor over a period of 110 days. The neutron uxemployed was estimated at 1.6)(1011 n/cm.2/second (over 1.6 mev.neutrons).

The method of exposure employed was to circulate the isopropylbiphenylat velocities ranging from 3 to 11 feet 5 per second through a loopsystem extending into the high flux zone lof -the neutronic reactor.

During the exposure of the monoisopropylbiphenyl to radiation, test datawere obtained which enabled the following properties to be determined.It should be stated that during approximately of the time of exposurecirculation had been unavoidably discontinued or was maintained onlysporadically s o that during such time a portion of the material wasexposed to more radiation than would have been the case had circulationbeen 15 maintained.

, The data so obtained follow:

l The compound isopropylbiphenyl has the structure:

As will be seen, the 2-carbon atom in the propyl chain is a tertiarycarbon atom. The single hydrogen atom, indicated in the structure aboveby an asterisk, which is attached to the tertiary carbonatornl is" averyreactive Characteristic Units Monoisopropylbiphenyl Specific GasGeneration Rate (radiolytie) Boiling Point Vapor Pressure Pour Point(melting point) C Density- Decomposed per 10-ls 30,

NVT (fast).

Kinematie Viscosity Thermal Conductivity Speelde Went 200 C 0 do 300 C.,0.637 Heat Transfer Coei. (h.) at 440 F. and 10 sec RTL/Hr., F., Ft.2 0.Induced Radioactivity, average Micro-euries 0.11. Flash Point (original)0.--- 139. Flash Point (after radiation) C 141. Fire Point (original) C152. Fire Point (after radiation)- O 155.

Effect of neutron radiation on hydrocarbons It is, of course, known thatwhen hydrocarbons are ex- 50 posed to neutron radiation collisionsbetween the neutrons and the hydrocarbon molecules form free radicals.This free radical forming reaction may be represented as follows: Y

where RR is a compound consisting only of carbon and hydrogen, R is afree radical derived therefrom, and R is either a carbon and hydrogencontaining free radical or the hydrogen radical H. n is a neutron. As aresult of employing either biphenyl or isopropylbiphenyl as the coolantor modifier in a neutronic reactor, free radicals of various types willbe present in the fluid as illustrated by the above reaction. The freeradicals may be hydrocarbon radicals or hydrogen radicals, and as iswell known such free radicals are very reactive and by combiningdirectly with the molecules of the particular moderator present formaddition compounds of higher molecular weight thus leading to tar or gasformation in many instances. Such addition compounds may comprisecycloalkanes, which are less stable than related aromatic compounds. Itis, accordingly, desirable to decrease the number'of free radicalspresent in the neutron modier as much as possible, since by so doing thetendency to form addition compounds is greatly diminished.v

hydrogen atom and possesses the property of reacting with free radicals.Accordingly, when the moderator consists in large proportion ofisopropylbiphenyl the free radicals generated by neutron bombardment aredecreased to a large extent by reaction with such reactive hydrogenatoms, thus decreasing the amount of damage to the hydrocarbons causedby vfree radical attack. Designating the isopropylbiphenyl by theletters IPBH, where H is the hydrogen atom, designated by means of anasterisk 'm the formula above, IPB is accordingly the isopropylbiphenylfree radical. The reaction by which free radicals are removed from themoderator uid probably takes place as follows:

which is a well-understood free radical exchange reaction. The productsRH and RH are hydrocarbon or hydrogen molecules, the molecular Weight ofsuch hydrocarbon molecule depending upon the size of the Original freeradical fragments. Such resul-ting hydrocarbons and hydrogen usuallyappear in the decomposition products of the moderator as gases. The freeradical designated by IPB- is a relativelyl stable free radical which,to some extent,A is discharged in part by reaction preferentially with ahydrogen atom or by reaction, to a lesser extent, with itself resultingin the formation of a dimer.

The above explanation, while in some respects based upon theory, hasfactual support. For example, the gases evolved from theisopropylbiphenyl, as a` result of neutron bombardment, have beenanalyzed and found to consist largely of mixtures of hydrogen andpropane. The specific radiolytic decomposition rate obtained during thetest, when circulation wasmaintained atapproximately the same rate ineach case, was.20 in the case of isopropylbiphenyl, as compared with 29in the case of biphenyl, thus indicating a considerably increasedradiolytic stability on the part ofthe isopropylbiphenyl.

Since isopropylbiphenyl is` readily oxidized, especially at elevatedtemperatures, it is necessary that oxygen be completely removed from thesystem and that this condition be maintained during operation.

Induced :radioactivity and control thereof Any organic liquid used asamoderator in a neutronic reactor, even though composed `exclusively ofcarbon and hydrogen, will acquireY some radioactivity. However, thiswill generally be of a very low intensity. Liquids which are circulatedthrough welded steel equipment will. pick up contaminants, such as ironscale or iiux residues, which subsequently acquire morel intenseact-ivity upon exposure to radiation. Contaminants can usually beremoved by simple filtration. It is, accordingly, desirable to insert asimple ilter at some point in the circulating fluid, by meansof which asmall portion of the liquid'is continuously ltered and'theV clean ltratereturned to the system.

I have found that certain hydrocarbon fractions have a pronouncedability to remove scale and iiux residues fromv iron and steel surfaces.A fluid which has an especially high descaling and cleaning property ismade by hydrogenating the normally solid hydrocarbons boiling above 350C. at 760 mm. pressure, which are formed during the synthesis ofbiphenylfrom benzene by pyrolysis at elevatedtemperatures. Suchhydrocarbon liquids are more fully described in Jenkins Patent2,364,719, issued December l2, 1944. The material described in saidpatent is referred to herein as HB-40.

The pronounced descaling and cleaning e'lecty on steel equipment wasshown in two tests `in which the eiectiveness of biphenyl and H13-40were compared. In the first test the interior of aweldedsteel tubularloop was rst cleaned and descaled, using hot caustic soda solution. Itwas then further cleaned by circulating hot biphenyl through the loop`for a few hours and then replacing. the contaminated biphenyl withsuccessive llings of biphenyl until the latter showed no further visibleamounts of scale. The loop was then insertedinto the neutronic reactorand exposed to neutron radiation. Radiation activtity tests Were' madelon samplesl of biphenly removed from th'e'loop. Thesesamples showed anactivity of 0.45 microcurie per gram `after a few hours of exposure tothe radiation. The biphenyl was retained in the loop forV over 100 hoursWithout evidencing any increase in activity above that originallymeasured.

After removal of the biphenyl from the loop and while the latter wasstill in the reactor it Was filled with HB-4O and the exposure toradiation continued. Samples taken from the materialin the loop showedan activity of 4.0 microcuries per gram of material. By simple ltrationof the sample the activity of the filtrate was reduced to 0.4 microcurieper gram.

The present discovery' affords a simple and convenient method forremoving contaminatingscale and impurities fromsteel or iron systems foranypurpose and particularly those which are to be. exposed to neutronradiation. It is generally necessary-only to iillthe system witlrtheliquid HB-40 andcirculate the same ata temperature above 100 C. andpreferably below 300 C. for a number of hours. The liquid is-thenremoved from the system; ltered and returned tothe system for furthercleaning. This treatment removes contaminants from the system about ltimes 'as eiectively as does biphenyl.

6 After the system had beenv cleaned, as described above, it was iilledwith isopropylbiphenyl, theV system irradiated by means of fast neutronsand a sample wtihdrawn for test. The induced radiactivity of theisopropylbiphenyl was only 0.11 mircocurie per. gram.

Use of isopropylbiphenyl as a moderator and coolant in a power reactor Atypical power reactor is illustrated diagrammatically in the ilow sheetshown in the accompanying drawmg:

In the drawing, numeral l0 indicates acylindrical reactor shellconstructed preferably of steel.' Within the shell 10 is arranged areactor core 11, which consists of plates of enriched uranium` of'suchnumber, size, shape and composition as to be capable of becomingcritical upon the addition ofy the isopropylbiphenyl. Surroundingthecylindrical shellltl is a cylindrical reflector shell l2, which is alsoconstructed of steell and whichv containsliquidreflector material. Inthe reactor core are inserted the usual control systems, indicated` bynumerals 52 and 53, the construction of which and use thereof beingvdescribed in the Fermi et al. patent, referred to above.

Numeral 13 indicates a disengager or gasrtrap, which is merely a devicefor separating gas from liquid. The disen-gager is connected with thereactor shell 10 by pipe 14. The gas Which is separated'from the liquidcoolant in i3 flows out by means of pipe 15, connected to pressurecontroller 16, which in turn is connected to condenser 17 by pipe linei8. Condenser 18 carries a discharge line or vent 19, permitting thedischarge of gases to the atmosphere.

Liquid coolant flows from-disengager 13 through line 20 into pump 21 bymeans of which the coolant is circulated into and through heat'exchangerorboiler 22 via line 23. Leaving heat exchanger 2'2. by pipe 24' thecoolant, now reduced-in temperature, isreturned to reactor shell 10 byline Z4.` BranchV lines 25carry the coolant into reflector shell 12 andthence by pipe 26 back into the main streamA owing intopipe M Pipe line27 carries a small stream of coolant from pipe 24 either into filter 28via pipe 29, thence returning the flow of iiltrate by pipe 30 t0 themain stream iiowing in pipe 24, or by means of pipes 31' and 32 intopuritication still 33. Heatingcoily 34 in the reboiler section of still33 provides thenecessary heat for distillation, the liquid returningthence to pipe 24 `by'means of pipe 35.

Liquid coolant which is fed to stil-l C43-flows Vthrough pipe 32 and`enters the still iirst passing pressure reducing valve 36, by whichmeans the flow is controlled to that nequired to keep the high boilingcomponents at the desired level. Still 33 may operate at substantiallyatmospheric pressure. The distillate in vapor form leaves the sti-ll bypipe 37 entering condenser 17, where the vapors are liqueed, the liquidresulting therefrom flowing through pipe 33 into pump 39 and beingthereby returned toy pipe 40 to the main stream llowingin pipe 20.Makeup liquid coolant is introduced into tank 4l and flows by pipe 42.into pipe 3S and thence into pump 39.

Purification still 33 may be operated continuously or intermittently asdesired. it is, of course, desirable to keep the high boilingdecomposition products in the circulating liquid as low as possible inview of the adverse effects of these products on viscosity and heattransfer. Small amounts of such high boiling decomposition productsusually in the neighborhood of 5-l0% by Weight of the liquid may betolerated without a substantial decrease in the heat transfer coeicient.After the high boiler content has reached a predetermined value (asdetermined by distillation ofthe sample) the purification still isplaced in operation and a constantk stream of coolant withdrawn from thesystem into the still`33 Where it is distilled. The distillate passesinto condenser 17,

where it is condensed and then returned to the system by means of pump39 as above described. The high boilers are removed from still 33 bymeans of pipe 45 containing valve 46 and thenceforth discarded.

Heat energy is withdrawn from the liquid coolant circulating in the heatexchanger or boiler 22 in any manner desired. In one method ofoperation, boiler feed water is introduced by means of pipe 50 and steamgenerated under pressure within boiler 22 being withdrawn at pipe 51 andsupplied to a steam turbine or other prime mover. The condensateproduced in the conventional condenser forming part of the prime moverwill again be returned to the boiler. It is, of course, not necessarythat water be used, since any thermally stable organic liquid of highboiling point may serve the same purpose and obviate the hazardsencountered with accidental leakn age of water into thecoolant-moderator system. The further utilization of the energy obtainedin this manner from a nuclear reactor is well known to those skilled inthe art and forms no part of the present invention.

In order to start up a newly constructed system, it is first cleaned outby conventional descaling agents and then treated by introducing H13-40fluid into supply tank 41 in amounts sufficient to fill the entiresystem. Pump 21 is placed in operation and circulation is maintained forseveral hours, the liquid withdrawn and visually examined forcleanliness. The uid should be filtered if contaminated and reintroducedinto the system and this procedure repeated several times until clear.The HIS-4t) is then completely withdrawn and the system filled with aninert gas such as nitrogen or helium, so as to eliminate traces of airand moisture pending the introduction of the isopropylbiphenyl charge.

The clean system is now loaded with isopropylbiphenyl by introduction tosupply tank 41, from which point it is permitted to ow into and throughthe pipe lines and varous pieces of equipment completely filling thesame with the exception of still 33 and condenser 17 which are notfilled. The system is filled to the point where the disengager isapproximately one half full. Pump 2l is activated, the control devicesin the reactor adjusted to release power in such an amount as to raisethe temperature of the isopropylbiphenyl in the system to a temperaturebetween 400 and 425 C. Heat is extracted from the heat exchanger orboiler in the manner described above and a point of heat supply from thereactor and removal at the boiler reached at which these quantities aresubstantially in balance.

Radiolytic damage to the fluid is evidenced by the accumulation of fixedgases in disengager 13 and also by the formation of high boilinghydrocarbons in the liquid. The fixed gases consist of a mixture ofapproximately equal volumes of hydrogen and lower hydrocarbons. As theamount of fixed gas increases in the closed system, the pressure risesto the desired value, after which it is continuously or intermittentlywithdrawn through pressure control valve 16. Withdrawal of gas ismentioned at such a rate so as to maintain the system under a pressurewhich is suciently high so as to prevent vapor formation in the hottestpart of the system. This hottest part of the system is adjacent to thefuel elements in reactor 11. Decrease of density occurring as a resultof increase in temperature will result in some loss of moderation byreason of the fewer hydrogen atoms per unit volume of coolant. Suchdecrease in moderation will, to some extent, damp out the nuclearreaction and can be compensated by adjustment of control devices. At allevents, it is necessary to maintain the gas pressure on the systemsufficiently high so that vapor formation will not occur.

The discharge of fixed gases attending the maintenance and theregulation of pressure upon the system will carry out someisopropylbiphenyl in vapor form. In order to recover suchisopropylbiphenyl the gases are discharged into condenser 17, whereinthey are cooled by contact with cooled surfaces maintained at a lowtemperature by means of cooling water. Condensed liquidisopropylbiphenyl will be returned to pipe 38, the valve therein nowbeing opened into the suction side of pump 39 and thence returned to thecirculating system.

The high boiling tar-like material formed concomitantly with the gasesby the effect of radiation should also be removed or maintained at adesirably low level. This is done by the withdrawal via lines 27, 31 and32 and reducing valve 36 of a constant stream of liquid flowing to still33. Still 33 operates under substantially atmospheric pressure as aresult of which the contents can be boiled by means of a side stream offluid passing to heating coil 34, located within the reboiling zone ofstill 33. The distillate leaving the still passes by line 37 also intocondenser 17. The condensate is mixed with that derived from thedisengager discharge vapors and is then returned by pump 39 to thesystem.

Removal of solid particles from the interior walls of the system whichbecome suspended in and carried by the circulating liquid is best doneby the provision of a filter 28 located in the system as shown in thedrawing. Such filter is supplied by lines 29 and the filtrate returnedby line 30 again to the system. The pressure drop across the filter maybe overcome by means of a suitable pump installed in either of theselines. By this means the induced radioactivity in the suspended foreignmaterials in the circulating fluid can be maintained at a low value.

Moderatng eectiveness of sopropylbiphenyl A measure of the moderatingeffectiveness of a hydrocarbon in a reactor is the number of hydrogenatoms contained in a unit volume of the substance. This measure iscommonly expressed in terms of one cubic centimeter of the hydrocarbonand is indicated by the symbol NH. A comparison of the NH values forbiphenyl and isopropylbiphenyl at elevated temperatures based ondetermined hydrogen values and densities shows that the latter compoundpossesses a moderating value which is from 21% to 25% greater than theformer. The values so determined follow:

NH VALUES Percent Temp. Biphenyl Isopropyl- Increase for biphenylIsopropylbiphenyl 3. 78)(1022 4.58)(1022 21 3.42)(1022 l.18 10l2 22 600F 3.04)(10 3. 80 102z 25 The increased moderating effectiveness of theisopropylbiphenyl means that a reactor utilizing such isopropylbiphenylcan be built correspondingly smaller than one built using biphenyl.

Specific heat, viscosity and melting point of isopropylbiphenyl andmixtures with biphenyl The specific heat of the material is a measure ofthe amount of heat required to raise the temperature of a given quantityof the material, and it will be appreciated that the higher thenumerical value of the specific heat of a given hydrocarbon, the lessmaterial is needed to remove a given quantity of heat. This property isof importance where heat is removed from the fuel elements in aneutronic reactor and delivered to a heat exchanger where it isextracted by another and cooler fluid. With this background it will benoted that isopropylbiphenyl possesses a substantially higher specificheat than does biphenyl. A comparison of the values for each compound isgiven in tabular form for several temperatures.

The viscosity of a liquid organic coolant is also of importance, sinceit determines the -heat transfer rate and also the amount of power whichWill be required for pumping the material through the system. It is, ofcourse, desirable to have a compound with a viscosity `as low aspossible. However, in the field of high boiling organic hydrocarbonliquids which Aare sufficiently thermally stable, the choice is narrowlyrestricted. We have discovered, however, that mixtures ofisopropylbiphenyl and biphenyl which have freezing points of 20 C. orlower and which have viscosity values also considerably lower than thatof pure isopropylbiphenyl, may be prepared from mixtures of biphenyl andisopropylbiphenyl. In the following table We show the composition ofseveral two-'component systems consisting of isopropylbiphenyl `andbiphenyl which are mutually soluble at temperatures at and below 20 C.and for these we give the measured viscosity of these mixtures as well-as the viscosities of the pure components.

Composition of Solution, Percent by Weight Solubility Kinematlc orMelting Viscosity, Point, C. Centistokes Isopropyl- Biphenyl at 210 F.

biphenyl Y What we claim is:

1. A neutronic power reactor in which the neutron moderator is ahydrocarbon material selected vfrom the group consisting ofisopropylbiphenyl and mixtures of isopropylbiphenyl and biphenyl, whichmixtures contain at least 80% by weight of isopropylbiphenyl.

2. A neutronic heating system comprising a liquid moderated neutronicreactor adapted to produce heat in an organic l-iquid, a heat exchangerand a liquid circulating system connecting said reactor and heatexchanger, and adapted to remove and transfer heat to a heat exchanger,said liquid system containing a hydrocarbon material selected from thegroup consisting of isopropylbiphenyl and mixtures of isopropylbiphenyland biphenyl, said mixtures containing at least by weight ofisopropylbiphenyl.

3. ln a neutronic heating system in which heat generated within a liquidmoderated neutronic reactor is removed from fuel elements therein andconveyed to a heat exchanger, the improvement which comprises effectingsaid removal and conveyance of heat by means of isopropylbiphenyl.

4, In a neutronic heating system in which heat generated within a liquidmoderated neutronic reactor is removed from fuel elements therein andconveyed to a heat exchanger, the improvement which comprises moderatingneutron ux Within said reactor `and removing heat therefrom by means of`a hydrocarbon material having a melting point not Iabove 20 C., saidhydrocarbon material consisting of isopropylbiphenyl andmixtures ofisopropylbiphenyl and biphenyl, said mixtures containing at least 80% byweight of isopropyl- 'biphenyl 5. The improvement defined in claim 4 inwhich the isopropylbiphenyl consists of `a mixture of the meta and paraisomer.

6. The improvement defined in claim 4 in which the isopropylbiphenylconsists of a mixture containing about 38% by weight of the para isomerand about 62% of the meta isomer.

References Cited in the iile of this patent UNITED STATES PATENTS2,364,719 Jenkins Dec. 12, 1944 2,482,904 Daugherty et al. Sept. 27,1949 2,556,128 Webb June 5, 1-951 2,708,656 Fermi et al. May 17, 19552,736,696 Wigner et al. Feb. 26, 1956 OTHER REFERENCES Bolt et al.: AECDocument ABCD-3711, March 15, 1955.

TID-5275, Research Reactors, pages 304-330. Published for use at GenevaConference Aug. 8-20, 1955, on Aug. 18, 1955. (Available -at ScientificLibrary.)

UNITED STATES PATENT OFFICE CERTIFICATE .F C0 ECHN Patent N00 2,902,425September lf, 1959 Milton Kosmn TEL,

Golunms 3 and ZW in che 'table-g under the, heading "Unitek, frs: linehereogP for "10""'378" read lOl we; same eolwmg, opposite "HestTransfert* Groei-fa {m} ai MOO Fo and 1G sew? for "BQTDQ" read s@ BTUm;@01mm 5,) line 53@ fer "biphenly" read s aipheny sw; eolwnn 7? line 57,for "mentioned" read E vInaunflz@ined wel,

signed and sealed this 10th day f May 19600 (SEAL.)

Attest:

KARL H., .AXLINE ROBERT C. WATSON Attesting Ofcer Commissioner ofPatents UNTTED STATES PATENT @FFME CERTIFICATE OF CORRECTWN Patent NoxQOQSKJ@ September ly 2.959

'lviflton Koemn el@ It is herebif certified that error appears in theothe above numbered patent re Patent should reed as corrected printedspecific-ation quiring correction and that the seid Letters below,

g Uniswp line Seme colump, opposite "eet @Uyl We; Taolwnn 7@ line 575,Afor Signed and sealed this lOth dey of Mey 19600 (SEAL.)

Attest:

KARL H0 MEINE ROBERT C. WTSUN Attesting Ucer Commissioner of Patents

1. A NEUTRONIC POWER REACTOR IN WHICH THE NEUTRON MODERATOR IS AHYDROCARBON MATERIAL SELECTED FROM THE GROUP CONSISTING OFISOPROPYLIPHENYL AND MIXTURES OF ISOPROPYLBIPHENYL AND BIPHENYL, WHICHMIXTURES CONTAIN AT LEAST 80% BY WEIGHT OF ISOPROPYLBIPHENYL.